Crystalline epoxy resins are used in fields that require reliable heat resistance, such as an electric/electronic field, because they commonly have rigid main chain backbones and are polyfunctional, and thus they are highly heat resistant.
However, only liquid compositions can work for certain uses such as cast molding, and thus crystalline epoxy resins are predominantly used for uses of solid materials, such as a transfer molding.
Also, epoxy resins used for liquid molding, such as cast molding, are liquid epoxy resins, which do not satisfy strict demands for improvements of curing characteristics, such as heat resistance, in recent fields of adhesion, cast molding, sealing, molding, lamination, and the like. Therefore, a demand for liquefaction of crystalline polyfunctional epoxy resins that give curing characteristics having high heat resistance has been raised. Meanwhile, a demand for stability of cured products in short wavelength regions, corresponding to white LEDs and blue LEDs, has also been raised.
Patent Document 1 describes a modified liquid epoxy resin obtained by reacting a crystalline epoxy resin with a compound that has two or more phenolic hydroxy groups and/or carboxy groups in a molecule and a compound that has one phenolic hydroxy group and/or one carboxy group in a molecule.
Patent Document 2 describes a method for manufacturing a liquid epoxy resin composition that is flowable in a room temperature, in which a mixture containing a crystalline epoxy resin and a liquid acid anhydride curing agent is melted and mixed for 10 minutes or longer at a temperature equal to or higher than the melting point of the epoxy resin to react the mixture to obtain a non-crystalline epoxy resin.
Patent Document 3 describes a tris-(2,3-epoxypropyl)-isocyanurate composition that is liquid at a room temperature, composed of: 100 parts by weight of the low melting point-type stereoisomer of tris-(2,3-epoxypropyl)-isocyanurate having a melting point of 98° C. to 107° C. and an epoxy value of 9.9 or more; and certain parts by weight of a curing agent, in which a ratio of a carboxylic anhydride to one epoxy group in a total epoxy resin is 0.5 to 1.5.
Patent Document 4 describes a method for preventing crystallization, which is for preventing solidification by recrystallization of a liquid composition containing a trivalent epoxy compound having a triazine nucleus as its backbone and an acid anhydride curing agent, characterized in that a moisture percentage in the composition is kept at 0.5% by weight or less.
Patent Document 5 describes a liquid epoxy resin composition in which a crystalline tris-(2,3-epoxypropyl)-isocyanurate having high solubility is reacted with an acid anhydride such as acetic anhydride. However, a manufacturing method of the tris-(2,3-epoxypropyl)-isocyanurate containing only α crystals, and not containing β crystals having low solubility is not described at all. In addition, in order to manufacture the tris-(2,3-epoxypropyl)-isocyanurate containing only α crystals, and not containing β crystals, recrystallization needs to be repeated to remove β crystals, which results in low recovery, and thus this is not preferable as an industrial manufacturing method.